It is possible, inter alia, to measure fill levels of a filler material in a container or an open raceway, for example, by means of such fill level measuring devices. In the process, a signal transmitted by an ultrasonic sensor, for example a brief ultrasonic pulse, is sent in the direction toward the filler material and is reflected at the surface of the filler material. The running time of the pulse from the sensor to the surface and its return is measured, and the fill level or the height of the filler is determined from this.
Such measuring apparatus is employed in many branches of industry, for example in the food industry, the water or waste water fields, and in chemistry.
For achieving as high as possible a transmission output of the ultrasonic sensor, the latter is preferably excited by means of its resonance frequency. As a rule, the resonance frequency is determined ahead of time, and thereafter the ultrasonic sensor is excited by means of this resonance frequency. However, the resonance frequency is not a constant value. For example, it is a function of the temperature of the surroundings of the ultrasonic sensor. Added to this is that, in case of industrial applications, media whose fill level is intended to be measured, can be deposited on the ultrasonic sensor. Such a deposit represents a mass, which is moved along by means of the oscillations of the ultrasonic sensor, and can therefore also result in changes in the resonance frequency.
Thus, in conventional fill level measuring devices the resonance frequency of the ultrasonic sensor is first measured at different temperatures and is stored in the device. The fill level measuring device is equipped with a temperature sensor, which is used to detect the actual temperature of the ultrasonic sensor. The resonance frequency assigned to the actual temperature is selected from the stored resonance frequencies on the basis of the actual temperature, and the ultrasonic sensor is excited by means of this frequency during the transmitting operation. This involves an additional outlay.
A possible change because of the formation of deposits is not detected by this.
In some applications it is possible, for example when a container is overfilled, that the ultrasonic sensor dips into the medium.
A method for measuring a fill level is described in DE-C 197 14 973 wherein it is determined by means of a post-pulse oscillation behavior of the ultrasonic sensor, whether the ultrasonic sensor is covered by the filler material.
If the ultrasonic sensor is covered by the filler material, no fill level measurement is possible anymore by means of conventional fill level measuring methods.